In the area of electronic circuit fabrication, it is necessary to bring discrete devices into electrical contact. For example, integrated circuits (or "chips") are often mounted on printed wiring boards, or other such devices, which may be generally referred to as substrates. The contact between the chip and substrate must have physical, chemical and electrical integrity and stability.
One method for physically and electrically connecting microelectronic devices employs the fabrication of metal pads on the top or exposed surface of various substrates. These metal pads are often formed with a top layer of solder; i.e., a low melting point alloy, usually of the lead-tin type, used for joining metals at temperatures around 450.degree. F. The solder pads are brought into contact with a metal structural element often referred to as a "metallurgy"--typically a metal pad--that will wet with liquid solder when heat is applied to join the solder and the metal pad and thereby form the electrical connection.
At present, most soldering processes comprise three basic steps: (1) pre-cleaning and deoxidation of surface oxides; (2) solder reflow and/or reflow joining; and (3) post-soldering cleaning. The pre-cleaning step is performed with different flux materials to prepare the surfaces for the soldering step by removal of contaminants and metal oxides from the solder surface. The solder joining step can occur only after the oxide coating is removed because the high melting point oxides will prevent the wetting of the two surfaces to be joined by reflow of the solder. Solder reflows into its characteristic spherical shape when heated, and joins the surfaces in contact with the solder. The third step, post-soldering cleaning, removes any flux residue remaining from the first step.
The post-soldering step has become more difficult to perform effectively as the size of electronic components shrink, making it much more difficult for the post-soldering cleaning agents to penetrate the smaller gaps between the components and the substrate. The post-soldering step becomes even more difficult when Surface Mount Technology (SMT) is employed.
Inefficient fluxing will result in defective bonding and inefficient post-soldering cleaning will reduce the long term reliability of the whole assembly. A high investment in cleaning equipment, materials, and processes can solve some of the problems, but undesired effects on the environment caused by cleaning solvents are generated.
A dry or fluxless soldering process can replace the pre-cleaning step and totally eliminate the post-soldering cleaning step. Since the main reason for using flux while reflowing solder joints is to break the high melting point and rigid oxide that covers the solder, a gas phase reaction that will remove this layer can replace the commonly used liquid fluxes that necessitate the post-soldering cleaning step.
Various attempts at fluxless soldering have been made; however, these attempts have suffered from limitations that made them applicable only to a small number and very specific applications.
For example, Moskowitz and Yeh in "Thermal Dry Process Soldering," J.VAC.SCI.TECHNOL.A, Vol. 4, No. 3, May/June 1986, describe a dry soldering process for solder reflow and bonding of Pb/Sn solder. This process uses halogen containing gases, CF.sub.2, CL.sub.2, CF.sub.4, and SF.sub.6 for the reduction of the surface oxide to enable solder reflow at temperature above the solder melting point. The activation energy needed for the oxide reduction by these gases is lowered by the use of a catalyst (Pt mesh) in a vacuum chamber. Yet the temperature needed for successful reflow bonding is 350.degree. C. This temperature is well above the typical soldering temperature for most electronic applications, i.e., 220.degree. C., and can damage the components, the substrate, and generate defects due to thermal mismatch between different materials.
Another attempt at fluxless soldering is disclosed in IBM Technical Disclosure Bulletin Vol. 27, No. 11, April, 1985, entitled, "Dry Soldering Process Using Halogenated Gas." The IBM bulletin discloses the use of halogenated gases in an inert carrier gas at elevated temperature to produce a reduction of solder oxide by the reactive gas and to allow solder reflow. Again, for the more common low temperature applications, thermal damage may result. Moskowitz and Davidson in "Summary Abstract: Laser-Assisted Dry Process Soldering," J. VAC.SCI.TECHNOL.A., Vol. 3, No. 3, May/June, 1985, describe a laser-assisted fluxless soldering technique for solder reflow. This technique uses laser radiation to excite an otherwise non-reactive gas in the presence of pre-heated solder surface. This technique requires direct access of the laser radiation to the solder surface, thus limiting the applications as well as resulting in a low throughput process.
Other attempts to remove surface oxides have employed sputtering. The sputtering methods, however, are extremely inaccurate and can damage the substrates and components while removing oxides, and are very limited in penetration distances, making sputtering unsuitable to applications like solder reflow.
In summary, the use of high temperatures in the available fluxless soldering methods may often have deleterious effects on the printed circuit boards and the components being joined. Laser assisted soldering methods have also proven inadequate for commercial use, because laser beams do not penetrate to unexposed areas, and thus cannot be applied to solder joining. In addition, being based on a localized beam, it is a time consuming process.